Ceramic moulded body

ABSTRACT

The invention relates to a ceramic moulded body, the surface of which is at least partly covered by a steel cladding, whereby the steel cladding is at least partly coated with a fire-resistant material on the surface thereof not covering the moulded body.

[0001] The invention concerns a ceramic form (workpiece), in particular a form made of a refractory ceramic material. Due to their fire-resistant properties, ceramic forms made of a refractory ceramic material can be used for applications where they are exposed to high temperatures, for example above 500° C., and even far higher.

[0002] For example, ceramic forms are used in metal casting, for instance as the pouring spout in sliding gate systems; for example, in continuous metal casting. Although the invention is not limited to the application of ceramic forms for a pouring spout (nozzle/tap hole) in sliding gate systems, it will be described hereinafter using such an application by way of example.

[0003] In metal casting, pouring spouts (also called nozzles or tap holes) in slide gate valve systems serve to convey the melt, for example molten steel, from the ladle into the intermediate container (“tundish”) or from the tundish into the ingot mold. For the necking down of the metal stream in these slide gate valve systems, the pouring elements can be designed in the form of “top hats” (plate with integral spout) or as interchangeable pouring spout. With the interchangeable spout, the spout is a part separate from the slide gate valve system, and thus interchangeable.

[0004] Ceramic forms in the shape of pouring spouts (nozzles) are usually enclosed on their outer surfaces with a sheet-steel covering. These help to improve the mechanical properties as well as the chemical resistance of the spout. At the high temperatures inherent in the casting process, the oxidizing atmosphere leads to the formation of oxides (scale) on the surface of the steel covering. This so-called scaling is due to the reaction of the metal sleeve with the oxygen in the air. With iron materials, the oxidation begins at above ca. 400° C. and it becomes especially active at temperatures above 600° C. The thickness of the scale layer increases with time and temperature. Should the scale layer spall off, be removed by mechanical means, or, as is usually the case under the stress of temperature changes, develops cracks, the scaling of the steel sleeve increases markedly. It can happen that the steel sleeve develops scale to the extent that it can no longer perform its intended function of improving the mechanical and chemical properties of the pouring spout.

[0005] Although by adding special alloying elements to the steel of the covering, such as create the forming of tightly adhering, dense layers of scale, it has been possible to slow the speed of the scaling process, such scale layers only possess low mechanical strength, and they can be easily removed from the steel sleeve, thereby exposing the unprotected underlying surface of the steel sleeve to further scaling.

[0006] In practice, therefore, scaling has mostly been countered by the use of buffer gases, especially argon, at high temperatures. Such a gassing of the steel sleeve, however, is cumbersome and expensive, and thus extremely uneconomical.

[0007] Moreover, argon gassing is not able to completely prevent the scaling of the steel sleeve.

[0008] As a result, scaling of the steel sleeve occurs even with argon gassing.

[0009] Where the scaling of the steel sleeve progresses too far, and as a consequence it is no longer able to effectively protect the ceramic form underneath, it then has become necessary to replace the pouring spout together with the steel sleeve, even though the ceramic form would permit a longer service life.

[0010] The invention has the objective of making available a ceramic form with its surface covered, at least partially, by a steel covering, that can be economically used, and on which the steel covering undergoes reduced scaling, even at high temperatures.

[0011] The invention arrives at the objective by means of a ceramic form whose surface is at least partially covered by a steel covering, where the steel covering, in that part of its surface that is not covering the ceramic form, is at least partially coated with a scale-inhibiting substance.

[0012] The reduction or prevention of scaling results in the mechanical performance of the metal covering is maintained for a longer time. This, in turn, means longer service life/durability for the ceramic forms.

[0013] It has been found that the oxidation (scaling) of the steel covering of a ceramic form, at those places where it is exposed to high temperatures in an oxidizing atmosphere (for example, air), can be effectively inhibited if the steel covering, at these places, is coated with a scale-inhibiting substance. In this connection, “scale-inhibiting” substances are understood to be substances, which, based on their composition, possess a comparatively higher resistance to scaling than the steel covering.

[0014] Substances that may be used to inhibit scale formation are, for example, metallic alumimum, chromium or silicon, or their oxides (Al₂O₃, Cr₂O₃, SiO₂), or other alloys or compounds thereof, singly or in combination.

[0015] The steel covering may also be coated with a scale-inhibiting substance in a manner such that, for example, metallic chromium or aluminum are applied to the steel covering, and are then “passivated” [[rendered passive]].

[0016] The scale-inhibiting substance can, for example, be applied to the steel covering as a solid, and, in the case of a metallic material, for example, as a sheet or plate. When sheeting is used, they can be made to adhere to the steel covering by, for example, welding or glueing them together. The sheet of scale-inhibitor can also be applied to the steel covering by shrink- or press-fitting. In particular, it is possible to arrange for the steel covering to be covered by the scale-inhibiting substance in a manner such that the steel covering, in those parts covered by the scale-inhibiting substance, cannot come in contact with the surrounding gas atmosphere. The lamination/coating, in other words, should be gas-tight.

[0017] In using a scale-inhibiting substance in sheet form, the facing surfaces of the scale-inhibiting sheet and the steel covering can be made to exactly correspond dimensionally, so that the scale-inhibiting sheet covers the entire surface of the steel covering. It may suffice, on the other hand, for the scale-inhibiting sheet to be attached in a gas-tight manner to the steel covering just along its edges, perhaps by welding.

[0018] An alternative version may have the scale-inhibiting substance applied to the steel covering as a liquid, for example, by brushing or spraying (e.g. with flame or plasma sprayers), or by having the steel covering impregnated with the liquid, with the liquid subsequently converted into a solid which coats the steel covering in a tight bond.

[0019] The scale-inhibiting substance may also be applied to the steel covering in powder form, and is subsequently tightly bonded with the steel covering.

[0020] It is generally sufficient to apply the scale-inhibiting substance to the steel covering in a thickness up to 1 mm, for example, in a thickness between 0.1 and 0.7 mm, or 0.2 and 0.5 mm.

[0021] As discussed earlier, the ceramic form can in particular be a ceramic pouring spout as an interchangeable spout, in the continuous casting of metals, and the following description will be based, by way of example, on such a discharge nozzle.

[0022] The surface of the steel covering facing the ceramic pouring spout can lie directly on the surface of the ceramic spout, and the steel covering may, for example, by applied by shrinking a previous heated steel sheet.

[0023] In one embodiment, the steel covering wraps around the outer surface of the spout. A version such as this is known in the present state of the art. Since the outer circumferential surface (surface) of the spout is usually formed with rotational symmetry, the steel covering can, in this case, be wrapped around the spout in the form of a sleeve.

[0024] The facing surfaces of the steel covering and the ceramic spout may be made to be matching in a way such that they come to lie against each other over their entire surfaces. In this case, the parts of the spout's surface that are covered by the steel covering are directly covered by the surface of the steel covering facing the spout. The mechanical strength of the ceramic spout can be enhanced by having the steel covering pre-stressed in its seat against the spout, for example by shrink-fitting. Alternatively, the steel covering may be provided for with only its edges in contact with the ceramic spout. The contact zone between the steel covering and the ceramic spout may be made gas-tight.

[0025] Further features of the invention may derive from the subclaims and other application documents.

[0026] There follows an expanded description of an illustrative version of a ceramic form according to the application, using the appended, highly schematized figure. Here, FIG. 1 shows a ceramic form in the shape of a spout for continuous metal casting, in a lateral cross section.

[0027] The pouring spout 1 consists of a ceramic form 3, covered in part by a steel covering 5. The steel covering 5 is covered in part by a scale-inhibiting substance 7.

[0028] The ceramic form 3 has rotational symmetry around its longitudinal axis L. Also with rotational symmetry around its longitudinal axis L, it has on its interior a channel 2 for the passage of the molten metal. An upper cylindrical section A of the ceramic form 3 adjoins below it a downward conically tapered section B, followed by another cyclindrical section C, and finally a conically tapering section D. The ceramic form 3 consists of a standard refractory ceramic material. A top face 9 of the ceramic form 3 has a recess 11.

[0029] On its outer circumferential surface 13, the ceramic form 3 is covered in part by the sleeve-shaped steel covering 5, which consists of sheet steel. The steel covering 5 lies fully against the outer circumferential surface 13. It starts at the upper edge 13 o of the outer surface 13 and ends at a distance from the lower edge 13 u of the outer surface 13 in the region of section D. That part of the circumferential surface 13 which extends in the area of section D from the lower end of the steel covering 5 to the lower edge 13 u is identified with the reference 13 f; it is the only section of the outer surface 13 that is not covered by the steel covering 5.

[0030] Moreover, the upper face 9 and the bottom face 15 of the ceramic form 3 are not covered by the steel covering 5.

[0031] The section of the steel covering 5 which covers section D of the ceramic form is covered by a scale-inhibiting aluminum layer 7. This aluminum layer 7 is placed (by shrink-fitting) as a sleeve over the steel covering 5, and forms a tight bond with it. This section represents the contact surface with a submerged entry nozzle or tapping spout, and is subject to the strongest oxidation. Accordingly, in this description of an illustrative embodiment, it is only this section of the steel covering 5 that is encased by a scale-inhibiting substance. 

1. Ceramic form (1), whose surface (9, 13, 15) is covered at least sectionally with a steel covering (5), where the steel covering (5) is covered, at least sectionally, in that part of its surface not covering the form (1), with a scale-inhibiting substance (7).
 2. Ceramic form (1) according to claim 1, as pouring spout for metal casting.
 3. Ceramic form (1) according to claim 2, as an interchangeable pouring spout.
 4. Ceramic form (1) according to claim 1, for a sliding value system.
 5. Ceramic form (1) according to claim 1, the steel covering (5) of which rests directly on the surface (13) of the ceramic form (1).
 6. Ceramic form (1) according to claim 1, the steel covering (5) of which is a sheet of steel.
 7. Ceramic form (1) according to claim 1, the steel covering (5) of which envelops the ceramic form (1) along its outer circumferential surface (13).
 8. Ceramic form (1) according to claim 6, the steel covering (5) of which envelops the ceramic form (1) under pre-stress tension.
 9. Ceramic form (1) according to claim 1, the scale-inhibiting substance of which is a metal or metallic compound.
 10. Ceramic form (1) according to claim 1, with a scale-inhibiting substance in the form of a sheet (7) or a layer applied to the steel covering (5).
 11. Ceramic form (1) according to claim 1, the scale-inhibiting substance of which consists of aluminum or an aluminum compound.
 12. Ceramic form (1) according to claim 1, the scale-inhibiting substance (7) of which is sprayed onto the steel covering (5).
 13. Ceramic form (1) according to claim 1, the scale-inhibiting substance (7) of which forms a gas-tight seal with respect to the covered surfaces of the steel covering (5). 